Organic pigments are required to be superior in terms of hue, hiding power, dispersibility, fluidity, solvent resistance, light resistance, weather resistance, heat resistance, water resistance and the like during use. Various treatments for obtaining pigments are carried out during or after synthesizing crude pigments to obtain a desired pigment, and the resulting pigments are used in various forms such as dry pigment, presscake or intermediates thereof.
Although various methods are used to produce dry organic pigments according to their synthesis method or finishing treatment, these methods can be broadly classified into those that go through finishing treatment accompanied by contact with organic solvent or water followed by filtration to obtain a solid in a wet state (referred to as a presscake or pressed cake and hereinafter to be simply referred to as a presscake) and drying. This same method is generally used to produce azo pigments, phthalocyanine pigments, quinacridone pigments and diketopyrrolopyrrole pigments.
For example, since azo pigments in the manner of soluble azo pigments or insoluble pigments normally undergo a coupling reaction in water, the form of the product after the reaction is an aqueous pigment suspension. This aqueous pigment suspension is then aged to carry out finishing treatment and then filtered for use as a colorant in the form of a presscake or in the form of dry granules or powder by further drying and pulverizing the presscake. The dried azo pigment is substantially free of water, thereby offering the advantages of not requiring management in the form of prevention of bacterial growth or preventing of freezing and the like and enabling lower transport costs per unit weight as compared with wet azo pigment.
Known examples of a method for obtaining such a dry azo pigment consist of drying a presscake as described above with a hot air heating type of drying apparatus in the manner of a box-type drying apparatus or air-permeable belt drying apparatus, followed by pulverizing to a desired particle diameter. However, although these methods are able to dry azo pigment to a moisture content of about 10% in a comparatively short period of time while consuming a small amount of energy, when drying is continued in an attempt to further reduce the moisture content to less than 2%, much more time and energy are required than drying to a moisture content of about 10%, thus resulting in the disadvantages of a long total drying time and considerable energy consumption.
Patent Document 1 discloses a method for drying an article to be dried with a spray drying apparatus so that the temperature of the article to be dried is 20 to 50° C. Since a spray drying apparatus rapidly dries from a state having a high moisture content to a moisture content of less than 2%, it has the disadvantage of consuming more energy than a hot air heating type of drying apparatus.
All of the examples of the prior art described above use a single drying apparatus to obtain a dry azo pigment by drying a presscake or slurry all at once.
In contrast, Patent Document 2 proposes a drying method comprising combining two different types of dryers and applying stepwise thermal hysteresis to an article to be dried. In this Patent Document 2, for example, a pigment is dried while pulverizing at a high temperature for a short period of time in a first step followed by further drying the dry pigment obtained in the first step in a second step at a lower temperature and for a longer amount of time than in the first step. In this method, a considerable amount of energy is consumed in the first step since the pigment is dried at a comparatively high temperature, and the pigment is dried at a lower temperature for a longer period of time than in the first step in the second step, thereby again resulting in the disadvantages of a long total drying time and consumption of a large amount of energy.
In the end, all of these methods of the prior art have the disadvantages of a long total drying time and consumption of a large amount of energy.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. S59-191765
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H7-278458